What does "Blue Hour" exactly mean? What time in the day can I capture this "Blue Hour" and how long is it? Also, what are its differences from the "Golden Hour"?
Golden hour is when the sun is low in the horizon, giving light with a golden tint.
Blue hour is when the sun is slightly below the horizon, leaving the sky deep blue.
There is no set duration because it varies according to latitude and time of the year.
Civil twilight is a good approximation with the sun being ±6° around the horizon.
When the angle is positive, you have the golden hour, when the angle is negative it is blue.
Often in English people use the term golden hour to include the blue and in french they use L'heure blue which means blue hour to include the golden. See this answer for more details.